关于Oracle中各个命中率的计算以及相关的调优
1)Library Cache的命中率:
.计算公式:Library Cache Hit Ratio = sum(pinhits) / sum(pins)
SQL>SELECT SUM(pinhits)/sum(pins) FROM V$LIBRARYCACHE; 通常在98%以上，否则，需要要考虑加大共享池，绑定变量，修改cursor_sharing等参数。
2)计算共享池内存使用率:
SQL>SELECT (1 - ROUND(BYTES / (&TSP_IN_M * 1024 * 1024), 2)) * 100 || '%' FROM V$SGASTAT WHERE NAME = 'free memory' AND POOL = 'shared pool'; 其中: &TSP_IN_M是你的总的共享池的SIZE(M)
共享池内存使用率，应该稳定在75%-90%间，太小浪费内存，太大则内存不足。
查询空闲的共享池内存:
SQL>SELECT * FROM V$SGASTAT WHERE NAME = 'free memory' AND POOL = 'shared pool'; 3)db buffer cache命中率:
计算公式:Hit ratio = 1 - [physical reads/(block gets + consistent gets)]
SQL>SELECT NAME, PHYSICAL_READS, DB_BLOCK_GETS, CONSISTENT_GETS, 1 - (PHYSICAL_READS / (DB_BLOCK_GETS + CONSISTENT_GETS)) "Hit Ratio" FROM V$BUFFER_POOL_STATISTICS WHERE NAME='DEFAULT'; 通常应在90%以上，否则，需要调整,加大DB_CACHE_SIZE
另外一种计算命中率的方法(摘自ORACLE官方文档<<数据库性能优化>>):
命中率的计算公式为:
Hit Ratio = 1 - ((physical reads - physical reads direct - physical reads direct (lob)) / (db block gets + consistent gets - physical reads direct - physical reads direct (lob)) 分别代入上一查询中的结果值,就得出了Buffer cache的命中率
SQL>SELECT NAME, VALUE FROM V$SYSSTAT WHERE NAME IN('session logical reads', 'physical reads', 'physical reads direct', 'physical reads direct (lob)', 'db block gets', 'consistent gets'); 4)数据缓冲区命中率：
SQL> select value from v$sysstat where name ='physical reads'; SQL> select value from v$sysstat where name ='physical reads direct'; SQL> select value from v$sysstat where name ='physical reads direct (lob)'; SQL> select value from v$sysstat where name ='consistent gets'; SQL> select value from v$sysstat where name = 'db block gets'; 这里命中率的计算应该是
令 x = physical reads direct + physical reads direct (lob)
命中率 =100 - ( physical reads - x) / (consistent gets + db block gets - x)*100
通常如果发现命中率低于90%,则应该调整应用可可以考虑是否增大数据缓冲区
5)共享池的命中率：
SQL> select sum(pinhits-reloads)/sum(pins)*100 "hit radio" from v$librarycache; 假如共享池的命中率低于95%,就要考虑调整应用（通常是没使用bind var ）或者增加内存
6)计算在内存中排序的比率:
SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)';—查询内存排序数 SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)';—查询磁盘排序数 --caculate sort in memory ratio SQL>SELECT round(&sort_in_memory/(&sort_in_memory+&sort_in_disk),4)*100||'%' FROM dual; 此比率越大越好,太小整要考虑调整,加大PGA
7)PGA的命中率:
计算公式:BP x 100 / (BP + EBP)
BP: bytes processed
EBP: extra bytes read/written
SQL>SELECT * FROM V$PGASTAT WHERE NAME='cache hit percentage'; 或者从OEM的图形界面中查看
我们可以查看一个视图以获取Oracle的建议值:
SQL>SELECT round(PGA_TARGET_FOR_ESTIMATE/1024/1024) target_mb, ESTD_PGA_CACHE_HIT_PERCENTAGE cache_hit_perc, ESTD_OVERALLOC_COUNT FROM V$PGA_TARGET_ADVICE; The output of this query might look like the following: TARGET_MB CACHE_HIT_PERC ESTD_OVERALLOC_COUNT ---------- -------------- -------------------- 63 23 367 125 24 30 250 30 3 375 39 0 500 58 0 600 59 0 700 59 0 800 60 0 900 60 0 在此例中:PGA至少要分配375M
我个人认为PGA命中率不应该低于50%
以下的SQL统计sql语句执行在三种模式的次数: optimal memory size, one-pass memory size, multi-pass memory size:
SQL>SELECT name profile, cnt, decode(total, 0, 0, round(cnt*100/total,4)) percentage FROM (SELECT name, value cnt, (sum(value) over ()) total FROM V$SYSSTAT WHERE name like 'workarea exec%'); 8)共享区字典缓存区命中率
计算公式：SUM(gets - getmisses - usage -fixed) / SUM(gets)
命中率应大于0.85
SQL>select sum(gets-getmisses-usage-fixed)/sum(gets) from v$rowcache; 9)数据高速缓存区命中率
计算公式：1-(physical reads / (db block gets + consistent gets))
命中率应大于0.90最好
SQL>select name,value from v$sysstat where name in ('physical reads','db block gets','consistent gets'); 10)共享区库缓存区命中率
计算公式：SUM(pins - reloads) / SUM(pins)
命中率应大于0.99
SQL>select sum(pins-reloads)/sum(pins) from v$librarycache; 11)检测回滚段的争用
SUM(waits)值应小于SUM(gets)值的1%
SQL>select sum(gets),sum(waits),sum(waits)/sum(gets) from v$rollstat; 12)检测回滚段收缩次数
SQL>select name,shrinks from v$rollstat, v$rollname where v$rollstat.usn = v$rollname.usn;
几个常用的检查语句
1. 查找排序最多的SQL:
SQL>SELECT HASH_VALUE, SQL_TEXT, SORTS, EXECUTIONS FROM V$SQLAREA ORDER BY SORTS DESC;
2.查找磁盘读写最多的SQL:
SQL>SELECT * FROM (SELECT sql_text,disk_reads "total disk" , executions "total exec",disk_reads/executions "disk/exec" FROM v$sql WHERE executions>0 and is_obsolete='N' ORDER BY 4 desc) WHERE ROWNUM<11 ;
3.查找工作量最大的SQL(实际上也是按磁盘读写来排序的):
SQL>select substr(to_char(s.pct, '99.00'), 2) || '%' load,s.executions executes,p.sql_text from(select address,disk_reads,executions,pct,rank() over (order by disk_reads desc) ranking from (select address,disk_reads,executions,100 * ratio_to_report(disk_reads) over () pct from sys.v_$sql where command_type != 47) where disk_reads > 50 * executions) s,sys.v_$sqltext p where s.ranking <= 5 and p.address = s.address order by 1, s.address, p.piece;
4. 用下列SQL工具找出低效SQL:
SQL>select executions,disk_reads,buffer_gets,round((buffer_gets-disk_reads)/buffer_gets,2) Hit_radio,round(disk_reads/executions,2) reads_per_run,sql_text From v$sqlarea Where executions>0 and buffer_gets >0 and (buffer_gets-disk_reads)/buffer_gets<0.8 Order by 4 desc;
更新时间：2013-04-21
